This invention relates to a self-propelled dolly for moving and elevating a load and, in particular, to a self-propelled dolly suitable for moving and maneuvering a mobile home into and out of its parking position.
In setting up a mobile home at its parking position or removing it, there can be difficult problems associated with elevating and maneuvering the mobile home within what may often be a relatively confined maneuvering area. To perform this function, it has previously been known to utilize a self-propelled dolly which can be guided beneath the mobile home and which has a lifting system for engaging and lifting the home at least partially off its wheels thereby enabling the home to be maneuvered in the elevated condition by motion of the dolly. One such prior dolly includes a generally rectangular frame provided with endless belt tracks of the caterpillar variety mounted along its opposite sides. Each track is driven by a drive sprocket engaging the interior of the track with a chain drive being provided between each drive sprocket and an associated one of two reversible electric motors. The prior vehicle included four vertical manual screw jacks at its corners, the jacks being joined at their upper extremities by cross beams supporting a load-carrying beam which could be moved into underlying supporting relation with the base of the mobile home.
Certain problems may, however, be encountered with a prior self-propelled dolly of the type described. One problem may arise from the absence of provision in the prior device for changing the speed of movement of the dolly in its load-carrying mode as compared with its traverse mode in which the dolly is maneuvered into position to assume the load or to move away after the load has been moved and disengaged. During the load-carrying movement, it is necessary to move the dolly at a very slow speed, much less than the walking speed of an adult human for example, in order to avoid imposing excessive forces on the mobile home structure and to avoid swaying or rocking the mobile home which might tilt it off its position of support on the dolly. On the other hand when the dolly is being maneuvered in the unloaded condition, it is necessary to have a rate of movement at least approximately equal to that of an operator walking adjacent to and controlling the dolly in order to avoid unnecessary waste of time.
It would be desirable, therefore, to have a self-propelled dolly capable of movement at approximately a human walking pace during its unloaded, traverse mode of operation and at a much slower speed (e.g. one-tenth of the traverse speed) during carrying of the load. To achieve this desirable result is far from simple as a dolly must necessarily be very low in height if it is to pass beneath a mobile home, which limits the complexity of the speed control structure which can be included in the dolly. Moreover, it is desirable to use reversible, synchronous electric motors so that both tracks can be run at the same speed to enable the vehicle to move easily in a straight line, thereby largely precluding the use of relatively and more expensive variable speed electric motors.
Another problem of the prior art device could arise in using the manually raised jacks positioned on the frame. Manual operation of lifting jacks is inherently slow in view of the large loads involved and the fine pitch of the threads which are necessary to operate manual jacks under such loads.
The load-carrying beam used with the prior art device could also present difficulties in that no provision was included for swiveling the beam relative to the direction of motion of the frame. Quite often, in maneuvering a mobile home, it is necessary to move the dolly supporting the home in a direction oblique to the center line of the mobile home. With the supporting arrangement of the prior device comprising a fixed transverse beam underlying the vehicle being transported, an attempt to move the frame obliquely to the mobile home, could impose such high torsional loads on the beam as to deform it or break its point of connection to the lifting jacks, thereby creating a hazardous condition with respect to the support of the mobile home of the dolly.